This invention relates to a catalytic in situ process for upgrading hydrocarbons in an underground reservoir. More particularly, it relates to a process in which a catalyst is placed along the horizontal segment of a horizontal production well operating in a toe-to-heel configuration, which enables carbon monoxide and/or hydrogen produced in the reservoir or injected into the reservoir with steam, to pass sequentially with reservoir oil over the catalyst, immediately prior to being produced.
Large supplies of heavy oil (ca. 8 to 15 degrees API) and medium oil (ca. 15-25 API) exist throughout the world, most notably in Venezuela, Canada and the U.S.A. Significant deposits are also to be found in the North Sea, China and Romania. The composition of the crude oil barrel is inexorably moving towards this heavier material, but it is relatively uneconomic to produce. Without upgrading to produce an oil of lighter composition, i.e. containing more white oil distillates (gasoline, diesel, etc.), heavy oil has very limited potential, historically trading at around $9.00 below the price of conventional oil. While surface upgrading technology (H-Oil, Vega-Combi or LC Fining processes) is possible, the payout time on the very large investment required is long (ca.ten years), which is discouraging to commercialization.
In situ oil upgrading has several advantages over conventional surface upgrading technologies. Because in situ upgrading (reaction occurring underground) can be implemented on a well-by-well basis, there is no need for large capital-intensive projects. Rather, the size of an in situ project for a particular field can be tailored to available production rates. Thus, in situ upgrading is practical even for those fields deemed too small to provide sufficient production for conventional surface upgrading processing. Additional advantages for in situ upgrading include the production of a more desirable and valuable product, ease in shipping and pipelining (minimum of 22 degree API gravity), and less demanding downstream processing (processable by a conventional refinery).
The requirements for an in situ upgrading process include: provision or a downhole bed of catalyst, achievement of appropriate high reaction temperatures and pressure at the catalyst bed, and mobilization of oil and co-reactants over the catalyst. Although the technologies to accomplish each of these tasks separately are known, their combination into a unified effective underground process has yet to be demonstrated in practice.
Because it is the most efficient method to achieve high temperatures in a reservoir without the direct application of heat, in situ combustion (ISC) is a promising oil recovery process to exploit in the development of an efficient and economical in situ upgrading process.
In-situ combustion processes are applied for the purpose of heating heavy or medium oil to mobilize it and drive it to an open production well for recovery. In general, the usual ISC technique used involves providing spaced apart vertical injection and production wells completed in a reservoir. Typically, an injection well will be located within a pattern of surrounding production wells. Air, or other oxygen-containing gases are injected into the formation. The mixture of air or oxidizing gas and hydrocarbons is ignited, a combustion front is generated in the formation and the resulting combustion front is advanced outwardly toward the production wells. Or, alternately, a row of injection wells may feed air to a laterally extending combustion front which advances as a line drive toward a parallel row of production wells.
In both cases the operator seeks to establish an upright combustion front which provides good vertical sweep and advances generally horizontally through the reservoir with good lateral sweep. However the processes are not easy to operate and are characterized by various difficulties. One such difficulty arises from what is referred to as gravity segregation. The hot combustion gases tend to rise into the upper reaches of the reservoir. Being highly mobile, they tend to penetrate permeable streaks and rapidly advance preferentially through them. As a result they fail to uniformly carry out, over the cross-section of the reservoir, the functions of heating and driving oil towards the production wells. The resulting process efficiency therefore is often undesirably low. Typically the volumetric sweep efficiencies are less than 30%.
Weissman et al (Energy and Fuels. 1996, 10.883,889) have recently proposed a modified in situ combustion process using a vertical well production strategy, in which a catalyst bed is emplaced around the production well. They have reported the results of two ISC tests, from which they concluded that a heated bed of hydrocracker catalyst placed in the bottom section of an 1.8 m long, 0.1 m diameter combustion tube (a special thin-walled reactor to simulate reservoir conditions), was effective in converting carbon monoxide, produced at the combustion front, and water, into carbon dioxide and hydrogen, via the water gas shift reaction. Not only was 50% of the sulphur removed by hydrodesulfurization (xe2x80x9cHDSxe2x80x9d), but there were also decreases in the oil density and viscosity obtained by the catalytic reaction. However, there were two main problems foreseen with this process: (1) the high retention of oil in the catalyst bed/gravel-pack (a large, one-place volume, with a vertical producer well), would lead to severe coking of the catalyst; and 2) the need to operate in either a cyclic mode (backflow on the injector well), or with a severe gas override condition to provide heat to the catalyst bed. Both processes possibly require supplementary downhole heating, which is both expensive and risky. Also, very importantly, the extremely acidic process gases (carbon dioxide and sulfur dioxide) entering the vertical production well, in combination with a temperature of over 300 degrees Celsius, will cause severe corrosion to the vertical well, through which all production fluids must pass. In the case of deliberate combustion-gas override (which is used to achieve high temperatures at the vertical well), very poor reservoir sweep will be achieved and the possibility of oxygen breakthrough is increased, which could cause a serious explosion hazard. In the case that the proposed cyclic combustion process is used (again to achieve high temperatures at the vertical production well), the production rates will be at least halved because of the required air injection time in the same well. In summary, the two proposed catalytic in situ processes appear expensive and difficult to operate, and so are not felt to be feasible.
A new viscous oil recovery process has recently been developed which provides a substantial increase in reservoir sweep efficiency over that of the traditional ISC process. A combination of wells is used wherein the toes of horizontal production wells are the first segments to provide hydrocarbon production and to come into contact with the injected gases. Greaves and Turta, in U.S. Pat. No. 5,626,191, disclose such a well configuration, which they call the xe2x80x9ctoe-to-heelxe2x80x9d oil displacement process. The patent applies to any process where gases are injected to reduce the viscosity of oil in an underground reservoir, and includes oxidizing gases for in situ combustion, steam injection, steam injection along with other gases, and hydrocarbon solvent gases.
Many oilfield operators are reluctant to apply the in situ combustion process because of historically poor performance when used in a traditional vertical well drive mode, and so an in situ upgrading process that uses steam would be more appealing. Since some heavy or medium gravity oil reserves are found in relatively deep and hot reservoirs, for example in the Orinoco Belt in Venezuela, high temperature steam can be utilized to provide the heat for an in situ upgrading process in some cases.
In summary of the prior art, it has been recognized as very desirable to achieve oil upgrading in an underground reservoir and the ISC process has been proposed as a possible heat source, but an effective process for implementing these steps has not yet been developed. The work underlying the present invention was undertaken to reach this objective. The present invention will now be described.
Considering the disclosure of U.S. Pat. No. 5,626,191 of Greaves and Turta that:
1. if a generally linear and laterally extending, upright combustion front is established and propagated high in an oil-containing reservoir: and
2. if an open production well is provided having a horizontal leg positioned low in the reservoir so that the well extends generally perpendicularly to and lies in the path of the front and has its furthest extremity (xe2x80x9ctoexe2x80x9d) spaced from but adjacent to the injection sources; then
3. the production well will provide a low pressure sink and outlet that functions to induce the lateral sweep front to advance in a guided and controlled fashion, first intersecting the toe and then proceeding along the length of the horizontal leg - under these circumstances, the oil displacement front will remain generally stable and upright and be characterized by a relatively high reservoir sweep efficiency; and
4. the unreacted injectant gases and reaction gases will flow through the swept portion of the reservoir and through the vertical reaction front and react with the oil at the front. Streamlines of the gases will bend towards the horizontal leg, due to the downward flow gradient created by the action of the production well as a sink, but will also rise due to gravity phase segregation, resulting in a net vertical front advancing laterally without significant over-riding. In the case of steam injection, the condensed water and heated oil will, along with any gases present, likewise flow down to the low pressure sink. In the case of in situ combustion the gases will be combustion gases: carbon monoxide, carbon dioxide, sulphur dioxide and water vapor.
Now, in accordance with the present invention:
if an appropriate oil upgrading catalyst is placed along the horizontal leg of a production well arranged in toe-to-heel configuration, including any of within the leg, on the leg or in the reservoir around the leg; then
hot combustion gases from an ISC process, or steam from a steam injection process combined with injected reducing gases, such as carbon monoxide or hydrogen, will react with the commingled oil over the catalyst at appropriate temperature and pressure and the oil will be substantially upgraded.
When compared in experimental runs with a conventional toe-to-heel (xe2x80x9cTTHxe2x80x9d) in situ combustion process in accordance with the Greaves and Turta patent, without catalyst in the vicinity of the horizontal leg of the oil producing well, the present invention was found to be relatively characterized by:
increased average API gravity of produced oil;
reduced oil viscosity;
reduced oxygen and carbon monoxide and increased carbon dioxide levels in the produced gas stream;
extensive hydrodesulfurization of the oil; and
extensive hydrodemetallization of the oil.
Additionally, the present process benefits from being a single pass catalytic process so that the reactant oil and gases continuously access fresh catalyst. The distributed catalyst along the horizontal well maintains high conversion activity by virtue of sequential catalyst exposure caused by the advancing movement of the combustion front from the toe to the heel of the horizontal well.
Therefore, in broad terms, the invention is a process for upgrading oil in an underground reservoir while the oil is recovered through a production well, comprising: providing an injection well for injecting a gaseous fluid into the reservoir to form an advancing, laterally extending displacement front operative to reduce the viscosity of reservoir oil; providing at least one open production well having a horizontal leg completed relatively low in the reservoir and positioned substantially perpendicular to and in the path of the advancing front; emplacing oil upgrading catalyst along the leg""s length; injecting the gaseous fluid into the injection well and advancing the displacement front along the leg; and producing the production well to recover upgraded oil from the reservoir.